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Practical Considerations for High Frequency Inductive Links Anthony N. Laskovski , Mehmet R. Yuce and Tharaka N. Dissanayake The University of Newcastle, University Drive, Callaghan, Australia; ABSTRACT Inductive power links are a popular method of wirelessly transferring power to small devices. High efficiency power transmitters such as the Class-E transmitter are the preferred choice for frequencies up to the low MHz range, however at frequencies above 100 MHz, the circuit does not produce a high enough efficiency. This paper investigates the consideration of parasitic elements of the transmission coils in the circuits, showing an improvement in the circuit’s performance. Keywords: Inductive Power Transfer, High Frequency, Self-Resonance 1. INTRODUCTION The latest trends in technology indicate that electronic devices are becoming increasingly sophisticated, less power hungry, smaller and more flexible, as evidenced by the surging development in mobile communications, personal devices, as well as medical telemetry and prosthetics. In fact, a number of interesting projects are taking place involving biomedical gadgets, many of which have been inspired by the Bionic Ear. Retinal Prosthesis has become a major field of research in the past twenty years and ideas such as wireless body sensor networks with implantable nodes and tongue-controlled devices.1 The size of a device dictates a number of factors, one of which is power, and research is constantly being conducted to improve the quality, efficiency and flexibility in supplying power to various electronic devices within a shrinking physical space. Batteries have been the traditional method of powering smaller devices, however they may take a considerable amount of room and require frequent replacement. This has led to investigations into wireless power transfer, which is the focus of this article. Figure 1 shows the basic blocks that will be discussed, from the power transmitter to the rectifier. One particular area of interest is the supply of power to implantable electronic devices. Power was traditionally supplied to implantable electronic devices such as Pacemakers by the use of Lithium ion batteries.2 Further author information: (Send correspondence to A.N.L.) A.N.L: E-mail: [email protected], Telephone: +61 249 217 842 M.R.Y: Email: [email protected], Telephone: +61 249 215 204 T.N.D: E-mail: [email protected], Telephone: +61 249 215 292 Figure 1. Basic blocks of inductive power transfer Table 1. Comparison of different powering methods for implantable devices Powering Method Device Frequency Energy Ref. Inductive Transfer & Battery Cochlear Implant 150 kHz 75 mAh 4 Inductive Transfer & Battery General 4 MHz 6.15mW 5 Direct Inductive Transfer Retinal Implant 1 MHz 100mW 6 Direct Inductive Transfer Animal Implant 4 MHz 4.1 mW 7 These batteries need to be surgically removed and replaced periodically due to discharging. A significant development in implantable power supply was the use of inductive coupling to charge an implantable rechargable battery. The concept may be understood by considering two windings of a weakly coupled transformer, where the core is a large air gap.3 The primary winding represents the transmission coil, and the secondary winding the receiving coil. Numerous applications have been developed along this wireless battery charging idea, varying in size, frequency and coil structure4 .5 The concept of wireless battery charging extended to solely wireless power transfer, which is a real-time powering system which saves a considerable amount of implant space, however it creates the requirement of supplying constant wireless energy to the implant. Table 1 gives an indication of different applications with their associated frequencies of operation and energy requirements. Considering that most new devices are small, little room is left for antennas. This increases the frequency required for any wireless links within a given space, which therefore increases the frequency required for a power transmitter. However, the transfer of energy becomes less efficient as the frequency of transmission increases,8 presenting a dilemma that size restrictions increase the possible frequency of operation, yet transmission efficiency is reduced as the frequency increases. Device complexity is also growing, and a greater demand is placed on achieving higher data rates, which require higher transmission frequencies9 .10 This forms yet another factor in selecting the correct operation frequency for a wireless device. Given the tradeoffs implied by these factors, it has been determined that in order to achieve more complexity without drastically compromising the power transfer efficiency, power and data must be transmitted in dual-band operation at low and high frequencies respectively96 .7 From the perspective of power transfer, Table 1 indicates that wireless power transfer is generally being implemented from the kHz to MHz range, however as the size of biological devices decreases, the power frequency band will simply have to increase. 1.1 Power Amplifiers Upon increasing pressure to transmit power at higher frequencies, efficiency becomes a more significant consideration, and in the area of high frequency transmission, switched-mode amplifiers are quite popular. The Class-D amplifier was a popular option for high frequency operation. It consists of a pair of p and n-channel transistors arranged as a voltage-switching inverter. The square-voltage output of this inverter is directed through a simple RLC band pass filter, R being the load. This amplifier was popular because of its high efficiency in practice, ranging from 70 to 80%, however its theoretical efficiency was 100%. The reason why the theoretical efficiency was unattainable, was because of the parasitic capacitance existent between transistor terminals, the effect of which increased with frequency.11 The Class-E power transmitter shown in Figure 2 was the next development in this field, and it has been successful for several reasons. It operates by switching its transistor while there is zero voltage accross C1 . Its robustness also comes from the fact that dv/dt at this point is also zero, meaning that very little energy is wastefully discharged1213 .14 The individual elements in the circuit also perform specific functions. C1 absorbs the parasitic capacitance between the transistor’s terminals, C2 blocks DC energy from reaching the load, L2 represents the primary side of an inductive link and R represents the load seen at the primary side of the transformer. L1 is simply an inductive choke. Basically, the transmitter was designed in such a way Figure 3. Small signal model of the Class-E power transmitter Figure 2. Class-E power transmitter that its load network created favourable switching conditions, and the circuit elements absorbed parasitic capacitance at higher frequencies. These aspects allowed for almost perfect efficiency to be achieved in practice, and it was introduced as a power transmitter for use in implantable electronics to become the current norm.15 Figure 3 shows a small signal model of the Class-E power transmitter shown in Figure 2. Expressing the load seen by the transistor in the Fourier domain will provide interesting information with regards to the circuit’s natural resonance. 1 1 || + sL2 + R ZL = sL1 || sC11 sC2 s2 L2 C2 + sRC2 + 1 (1) = 2 + sL1 1 s3 L2 C1 C2 + s2 (RC1 C2 ) + s C1 + LL2 C1 2 + C2 + RC L1 If the choke inductor L1 is assumed to be large, the resonance of the circuit is expressed as (2) 1 ω= p L2 C1 ||C2 (2) 1.2 Energy Transfer The inductive transfer of energy forms a vital element of supplying power to a secondary device, and it is a natural point of interest to focus on efficient energy transfer, especially with regards to the self-resonance of inductive transfer coils. A thorough analysis has been conducted, where inductive wireless links for implantable electronics have been modelled.16 The analysis shows that the self-capacitance and self-resonance of multi-strand inductors play a significant factor in determining the Q factor of inductors. Hardware results supported these findings in the low MHz region. 1.3 Rectification The next phase of energy transfer is to rectify the received power and supply the device with DC energy. In higher power applications, this is as straightforward as implementing a diode bridge and a voltage regulating unit, however achieving this at higher frequencies with lower power levels is difficult. The first obstacle is the diode, which is the basic building block of any rectifier. It has an immediate voltage drop ranging from 0.1 to 0.7V, depending on the type of diode, meaning that it is favourable to use less diodes in most medical inductive transfer applications5177 . As mentioned in Section 1.1, the highest frequencies for inductive transfer are currently in the order of MHz, and most designs send more power to compensate for the forward voltage drop of rectification diodes.3 An interesting development in this field has been the Class-E rectifier. It is designed for high efficiency operation at high frequencies due to its zero switching properties. The Class-E rectifier is mostly used in DC/DC converters as shown in Figure 418 .19 Figure 4. Class-E DC/DC Converter18 Figure 5. Active Diode20 Figure 6. Class-E power transmitter with parasitic capacitor C3 Figure 7. Small signal model of a Class-E power transmitter with parasitic capacitor C3 Another concept aims to remove the voltage drop on a rectifier all together by using comparators to sense the initiation of the diode’s forward-bias, then sending a one-shot pulse to a switch, allowing the energy to bypass the diode and supply the load, as shown in Figure 5. The concept has been tested, with an input voltage signal at 5 MHz and the target implant supply voltage set at 3V with variable loads of 2 kΩ and 10 kΩ. The results from these investigations show up to a 70% increase in received power compared with an on-chip passive rectifier.20 2. PRACTICAL CONSIDERATIONS 2.1 Power Amplifiers It was mentioned in Section 1.1 that a trend of decreasing size in implantable devices will increase power transfer frequency. While the Class-E power transmitter has proven to be very successful at the highest end of power transfer frequencies, there may be limitations as to how well the transmitter will perform at even higher frequencies. As the frequency of operation increases for the Class-E power transmitter the size of capacitors and inductors decreases, and at frequencies above 100 MHz typical capacitor sizes are in the low pF region. Higher operation frequency also gives rise to more influential parasitic capacitance across the inductive transfer windings, and the traditional equations used to determine peak switching conditions are no longer suffice12 .14 If the parasitic capacitance is introduced to the Class-E transmitter, it would appear as C3 in Figure 6 . In a similar approach to Section 1.1, the load network was expressed in the Fourier domain so as to obtain an expression for the circuit’s resonant frequency. Table 2. Hardware results of Class-E power transmitters at different frequencies with different Inductors Frequency Traditional Class-E Parasitic Consideration 20MHz 60mW 80µW 133MHz 11mW 85mW 403MHz 45µW 112µW 1 1 1 || + ||(sL2 + R) sC1 sC2 sC3 s3 L1 L2 (C2 + C3 ) + s2 RL1 (C2 + C3 ) + sL1 = 4 (C1 C2 + C1 C3 + C2 C3 )(s L1 L2 + s3 RL1 ) + s2 (L1 C1 + L1 C2 + L2 C2 + L2 C3 ) + sR(C2 + C3 ) + 1 (3) ZT = sL1 || If the choke inductor L1 is again assumed to be large, the resonant equation of the circuit appears as (4). 1 ω=p L2 (C1 ||C2 + C3 ) (4) Comparing this expression with (2) shows that (4) is a more detailed expression for the circuit’s resonance. If the circuit is simplified to exclude parasitic capacitance, the resonant frequency expression will revert to (4). The consideration of the transmitting inductor’s parasitic capacitance lead to the question of how to use the Class-E transmitter at frequencies above 100 MHz. Given that the parasitic capacitance of the transmitting inductor plays a more important role in the Class-E power transmitter as frequency increases, it may imply that the self-resonance and parasitic element of the inductor is also worth considering at these frequencies. Three Class-E transmitters were implemented in hardware to investigate this point of interest. One was designed to operate at 20MHz and the others at 133 MHz and 403 MHz, each according to established prescriptions.14 The output power was recorded by measuring the voltage accross the resistor R shown in Figure 2, and the curves are presented in Figures 8, 10 and 12. Photos of three of the inductor coils are shown in Figures 14, 15 and 16. ... including their parasitic capacitance... The circuits were then redesigned by replacing the existing inductive coils, including their parasitic capacitance, which was determined by (5), where εo is the permeability of free space, r is the radius of the coil, a is the thickness of the wire, N is the number of turns and l is the length of the wire.21 C= εo 2πrN.2a l/N (5) √ The inductance was also calculated22 and the resonant frequency determined by ω = 1/ LC. The original inductors were replaced with the inductors, self-resonant at the circuit’s operation frequency. The output signal produced by the 20 MHz, 133 MHz and 403 MHz circuits are shown in Figures 9, 11 and 13 respectively. A comparison of these results in Table 2 indicates that the 20 MHz circuit produces more power using the original inductor as specified by Class-E design principles when compared with the inductor optimised for its self-resonance at the operation frequency. The 133MHz circuit on the other hand produces a lower output when using the prescribed Class-E inductor, compared to the output achieved when the parasitic capacitance is considered to design a self-resonant inductive coil. The 403 MHz circuit produced considerably lower power, however the power of the circuit designed to consider parasitic capacitance was higher. A possible reason for this was the use of a lower power transistor SOT-343 for the 403 MHz experiments. Figure 8. Voltage accross 10 Ω resistor in a Figure 9. Voltage accross 10 Ω resistor in a 20MHz Class-E power transmitter with tradi- 20MHz Class-E power transmitter with self restional specifications onant inductor Figure 10. Voltage accross 10 Ω resistor in a Figure 11. Voltage accross 10 Ω resistor in a 133MHz Class-E power transmitter with tradi- 133MHz Class-E power transmitter with self tional specifications resonant inductor Figure 12. Voltage accross 10 Ω resistor in a Figure 13. Voltage accross 10 Ω resistor in a 403MHz Class-E power transmitter with Tra- 403MHz Class-E power transmitter with self ditional specifications resonant inductor Figure 15. A coil used for 133 MHz inductive transfer Figure 16. A coil used for 403 MHz inductive transfer Figure 14. A coil used for 20 MHz inductive transfer 3. CONCLUSION This paper outlined a number of developments in the area of inductive power transfer, from power transmitters to rectification. Considerable focus was placed on the use of power transmitters at higher frequencies using the Class-E power transmitter. It was determined that it is important to consider parasitic elements in the Class-E power transmitter when using it as an inductive power transmitter above 100 MHz. These considerations will improve the output power of the inductive links at higher frequencies in addition to assisting in the reduction of device size and perhaps the transmission distance. 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